Manufacture of wood pulp



July 19, 1932. o, KR'ESS l 1,861,626 MANUFACTURE OF WOOD PULP Filed April 2. 1927 Patented July 19, 1932 A i I 1,867,626 i UNITED STATES PATENT oFr-lcE OTTO KRESS, 0F APPLETON, WISCON S111' MANUFAc'runE or woon :PULP

Application led .April 2, 1927. Serial No. 180,407.

The invention relates to the manufacture ufacture of kraft pulp, it would be particuof pulp for the making of paper and the like, larly diificultvand impractical to bring back and more particularly to the manufacture of all of the excess liquor withdrawn from the pulp by the sulphate or kraft process. charge after impregnation, to the proper 5 The chief object of the invention is to prostrength and the proper proportions of the 55 duce a more uniform and economical digesactive chemicalscaustic soda and sodium sultion of the wood by impregnating or saturatphide, because of the relatively greater abf ing the wood chips with the akaline liquor sorption of the caustic by the wood. before cooking, and carrying on the process In accordance with the present invention,

1o in such amanner that it is commercially practhe wood chips, previous to cooking, are imou tical and eicient. pregnated with hot alkaline digestion liquor It has heretofore been proposed to impregunder pressure, but deficiency in the active nate the wood chips with the hot alkaline chemical ingredients is avoided by withdrawliquor under pressure and then, after withing only a portion of the excess liquor after C drawing the .excess liquor, to subject the impregnation, so that the charge is then charge to cooking temperatures and effect the cooked in the presence of free or unimpregdigestion of the wood by the chemical connated liquor of such amount that the chemit-ained in the impregnated chips. But it is cal contained 'thereinY and in the impregnated believed that such processes have not-met with chips is sulicient to properly pulp the wood.

*n commercial success for, with liquors of suit- Also, the active chemical ingredients of the 70 able strength for economical and successful mpregnating liquor are proportionedrto mill practice, the chips will not absorb sucompensate for the relatively greater absorpcient chemical to produce a proper pulping tion of the caustic in the impregnating opaction. In particular, there will be avery deeration and such that suilicient sulphide is cided deficiency in sodium sulphide in the left in the digester. By withdrawing apor- 75 impregnated chips if the manufacture of tion only of the excess liquor after impregkraft pulp is attempted in this way, since nation, not only is sucient chemical rethe caustic soda is not only absorbed mechanitained in the digester for the cooking operacally, but also some of it unites chemically tion, but the amount which must be strength- 31 with the wood during the impregnating opened for re-use is materially reduced.

eration and hence it is taken up by the chips Also, in accordance with the present invenin relatively greater proportion than the sultion, the impregnating liquor employed has phide. Also, because of the chemical absorpa relatively lower proportion of caustic than tion of the caustic, the excess 'liquor withthe available white liquor and such that- 7 drawn from the charge will be of such large the liquor Withdrawn from the digester after amount and so Weak in caustic that it canimpregnation can be brought back substannot be economically strengthened for re-use tially to uniform strength and to substantialand, in particular, cannot be brought up to ly uniform proportions of its active ingredithe desired strength with the available white ents, by the addition thereto of an amount i" liquor recovered from the operation, since, of available white liquor that is substanin commercial practice as is `well known, this tially equal to or less than the amount of liquor is necessarily of limited strength; This vliquor left in the digester. ln case the white is because the soda is recovered from the liquor required for this purpose is less than black liquor resulting from the digestion the amount of liquor left in the digester, the 17' process, by vevaporation and incineration, in difference in bulk is made up with water. $5 the form of carbonate and is converted or Any suitable apparatus may be used, but partially converted into caustic by lime; and, preferably some such arrangement as shown since this causticizing reaction is reversible, in the accompanying drawing is employed, y it is not commerclally practical except in soand preferably the digester is of the rota 0" lutions of llmited strength. A lso,inthe mantype and thev cooking is effected by the in- 10 jection of steam. The improved process for the manufacture of kraft pulp is carried out substantially as follows: l

The chips of from 9%; to 7A; of an inch long with the grain, arecharged through an opening in one end of the digester 1 from a supply hopper 2, and at the same time hot alkaline cooking liquor from an impregnating tank 3 is run into the digester through a pipe 4 to pack down the charge. The digester opening is then closed by a cover plate 5 and additional liquor is drawn from the tank and forced into the digester by a pump 6. The impregnating liquor is hot and preferably at about a temperature of 165 degrees F., asteam coil being provided, if necessary, in the impregnating tank for this purpose. A pressure is also maintained by the pump, preferably of about 135 pounds, for 15 to 2O minutes, to thoroughly impregnate the wood chips with the hot liquor. Other means, such, for example, as the introduction of air or steam into the upper portion of the digester could be employed for applying pressure to the liquor during the impregnating operation, but a pump is convenient both for forcing the liquor into the digester and also for withdrawing part of the liquor therefrom.

Before cooking, a portion only of the eX- cess liquor is withdrawn by the pump, which is of the reversible type, and is returned thereby to the impregnating tank. The charge is then cooked while the digester'is rotated in the usual manner, and preferably by the introduction of' steam of 125 to 135 pounds pressure through a pipe leading through one of the digester trunnions, for a Y 7. Preferably also, the impregnating liquor period of about two hours. Then the pulp is blown from the digester and washed and @the resulting black liquor is treated in the usual way for making fresh white liquor. As stated, only a portion of the excess or unimpregnated liquor is withdrawn from the digester and returned to the tank, and such as to leave from 17 to 22 pounds of active chemical per pounds of dry wood in the digester, and substantially in the proportions of two parts of NaOH to one part of NagS. The amount of chemical left inthe charge depends upon the kind of wood, its moisture content and the desired chaiacter of the pulp.

Also, as noted, the impregnating liquor con-r` tains a substantially lower proportion of vcaustic than the available white liquor, Y and such that the liquor withdrawn from the digesterand returned to the impregnating tank can bebrought back sustantially to uniform strengthby the addition thereto of the available white liquor`from a supply tank is as high in content of caustic as will permit this strengthening and such that the return liquor can be brought back to its original strength and amount by the addition of an amount of white liquor equal to the amount of liquor left in the digester.

Thus, if the available white liquor contains about 10 per cent by weight of caustic soda and 5 per cent by weight of sodium sulphide, the impregnating liquor will preferably contain about 6 per cent of caustic and 4.5 per cent of sulphide. In treating tamarack, hemlock and jack-pine, in the manner described with such a liquor for the manufacture of sulphate pulp, about 40 gallons of liquor of a temperature of about 165 degrees F., for each 100 pounds of dry wood `will be run into the digester as it is filled with chips, and then about 10 gallons additional for each 100 pounds of wood will be pumped in, or until a hydrostatic pressure of about 135 pounds is developed and maintained for a period of 15 to 20 minutes, to

thoroughly impregnate the chips, so that l the digester will contain about 50 gallons of liquor having about 25 pounds of NaOH and about 18.76 pounds of Na2S or about 43.76 pounds of total active chemical, for each 100 pounds of dry wood contained in the charge. Then for each of 50 gallons of liquor which has been placed in the digester, about 34 gallons are withdrawn and returned to the impregnating tank, leaving about 16 gallons per 100 pounds of charge left in f the digester. But, because of the chemical absorption of the caustic by the wood, the proportion of this ingredient is materially decreased in the withdrawn liquor, and it contains only about 4 per cent Iof caustic and 4.3 per cent of sulphide, so that each 34 gallons of return liquor contains 11.34 pounds of NaOH and 12.19 pounds of NazS. Hence there is left inthe digester 25 minus V11.34

or 13.66`pounds of NaOH land 18.76 minus i 12.19 or 6.57 pounds of NazS, making a total of 20.23 pounds of active chemicals for each 100 pounds of dry wood in the charge, and in the proportions of substantially two parts of caustic to one part of sulphide. Then by addingto each 34 galpounds per 100 pounds of dry wood, and

in the proportions of'about two parts of caustic to one part of sulphide. Of these active chemicals, however, the impregnated chips will contain the greater proportion of the caustic, andthe free or um'mpregnated liquor, the greater proportion of the sulphide. lf all excess or free liquor were withdrawn, there would be, as stated, a deficiency in caustic and such a marked deficiency in sulphide that satisfactory kraft pulp could not be produced. It is particularly noted that if the wood were at all wet it would absorb A substantially the same amount of caustic as if it were dry, but the amount of sulphide absorbed would be very materally reduced. The present improved process, however,

avoids any difficulty in treating either dry or wet wood.

. The iguresabove noted will of course vary with the strength of the available white liquor, the character of the wood treated and the pulping action which is desired'. In good commercial practice, the white liquor will contain caustic and sulphide in the proportions of two parts of caustic to one of sulphide. Also it will vary in its content of active chemicals from 8.4 to 11 per cent of NaOH and from 4.2 to 5.5 per cent of NazS, and the impregnated liquor mayr vary from about 5.4 to 6.3 per cent of caustic and from about 4 to 5.7 per cent of NaZS, or from 9.4 to 12 per cent of total active chemical. As stated, it is preferable to use as strong mpregnating liquor as'will permit the proper strengthening of the return liquor in the manner described. In this way the undue weakening of the resulting black liquor is avoided and the cost of evaporation decreased. A relatively stronger impregnating liquor will proportionately decrease the amount of free liquor which is left in the digester for the cooking 'operation, and a relatively weaker impregnatng liquor will require that a proportionately greater amount be retained in the charge and will usually require water in addition to the available white liquor in bringing it back to substantially uniform amount and proportions of its active ingredients.

ln practice, the return liquor and the white liquor are tested to determine the strength of the active chemicals therein and the amounts of these liquors passed into the impregnating tank are regulated accordingly to leave in the digester the amount of chemicals required for the pulping operation and to restore the impregnating liquor to'substantially uniform strength and amount. For convenience in thus handling these liquors, the mpregnating tank is.provided with a Hoat actuated gauge. Itis also noted that the amount of liquor contained in the impregnating tank at the beginning of any cycle of operations should be substantially the same and somewhat in excess of that required to effect the impregnation of the charge.

The improved process has been carried out commercially in the manufacture of sulphate pulp and it has been found that, because the cooking of the chips is more uniformly effected, the yield and strength of the pulp are materially increased and also that the time and amount of chemicals required are reduced, and that as stated, it is equally satisfactory for the pulping either of Wet or dry wood. It is also noted that in accordance with the improved process, the impregnating liquor is such that the required amount of chemicals for the pulping operation is left in the'digester for each cook and that the liquor Withdrawn after the impregnating operation can be brought back to substantially uniform. lstrength and amount with available white liquor and, if required, with the addition of water. Hence the process can be repeated indefinitely in a commercially eiicient manner and without the accumulation of excessive amounts of weak return liquor which could not be economically utilized.

Obviously, changes may be made in the details set forth Without departure from the scope of the appended claims.

l claim as my invention:

1. The method of making kraft wood pulp which comprises impregnating. a charge 0f wood before cooking with hot liquor under pressure and which impregnating liquor contains substantially from 5.4 to 6.3 per centof sodium hydroxide and substantially from 4 to 5.7 per cent of sodium sulphide, withdrawing a portion only of the excess liquor and of such amount that substantially from 17 to 22 pounds of active chemicals per 100 pounds of wood is left in the digester and in the proportions substantially of two parts of sodium lo@ hydroxide to one part of sodium sulphide, then cooking the charge, and restoring the Withdrawn impregnating liquor to its original strength and amount by the addition of recovered white liquor containing suhstanim tially fromSA to 11 per cent of sodium hydroxide and from 4.2 to 5.5 per cent of sodium sulphide substantially in the proportions of two parts of sodium hydroxide to one part of sodiumV sulphide, substantially as de- M scribed.

2. The process of preparing sulphate or kraft pulp in successive cycles of operation, which comprises impregnating each digester charge before cooking with hot alkaline liqu uor under pressure of substantially 90 pounds per square inch or more and containing substantially Vfrom 9A to V12 per cent of active chemicals in a proportion of substantially from 1.2 to 1.3 parts of sodium hydrom'de to la@ one part of sodium sulphide, withdra. a portion only of the free liquor after impregnation and of such an amount that from 17 to 22 pounds of active chemicals per 100 pounds original strength, amount and proportionsof 130- of dry wood are left in the digester and in active chemicals for the subsequent operation by the addition thereto of an amount of recovered white liquor substantially equal to the amount of impregnating liquor left in the digester and containin substantially from 12.5 to 16.5 per cent o active chemical and substantially in the proportion of two parts o sodium hydroxide to one part of sodium s hide, substantially as described.

3. The process of preparing sulphate or kraft pulp in successive cycles of operation, which comprises impregnating each digester charge before cooln'ng with hot alkaline liquor under pressure of substantially pounds per square inch or more and containing substantially 10.5 per cent of active chemicals in the proportions of substantially one and one quarter parts of sodium hydroxide to one part of sodium sulphide, withdrawing av portion only of the free liquor after impregnation and of such an amount that substantially 20 pounds of active chemicals per 100 pounds of dry wood are left in the digester and substantially in the proportions of two parts of sodium hydroxide to one part of sodium sulphide, then cooking the charge and restoring the withdrawn impregnating liquor to its orlginal strength, amount and proportions of active chemicals for the subsequent operation` by the addition thereto of an amount ofvrecovered white liquor substantially equal to the amount of liquor left in the digester and containing substantlally 15 per centof active chemicals and substantially in the` proportions of two parts of sodium hy# droxide to one part of sodium sulphide, subq stantially as described.

. OTTO KRESS. 

